Density of components on integrated circuits is constantly increasing in the semiconductor industry. As a result of the increase, masks used in photolithography are required to project structures of ever decreasing size onto a photoresist layer on a wafer. The wavelength of the radiation used in exposure of the photolithographic masks must be sufficiently short in order to meet this requirement. Typical exposure wavelengths have been shifted into the far ultraviolet region of the electromagnetic spectrum. Typically at present, a wavelength of 193 nm deep ultraviolet (DUV) radiations is used for the exposure of the photoresist on wafers. In some cases, wavelengths in the extreme ultraviolet (EUV) range of the electromagnetic spectrum (e.g., 13.5 nm) are used. The complexity of the manufacture of photolithographic masks with increased resolution is increasing together with the expense.
Photolithographic masks are expected to conform to stringent demands and tolerances with respect to transmission homogeneity, planarity, purity, and temperature stability. For example, the tolerable deviation of a mask substrate from planarity is only a fraction of the exposure wavelength. Larger deviations may significantly disturb the phase front of the electromagnetic wave reflected from a multilayer structure on a surface of the substrate. Deviations of the planarity of the substrate of the photolithographic mask may lead to variations of the optical intensity distribution on the photoresist due to constructive or destructive interference. During further processing of the wafer, the variations in optical intensity may result in defective fabrication of semiconductor devices.
Decreasing exposure wavelength makes this problem more challenging. For example, with EUV radiation the allowed deviation is on the order of nanometers. A substrate as supplied by a manufacturer may not meet the planarity requirements for EUV photolithographic masks. The process of fabricating the mask, which includes formation of fine patterns on a surface of the substrate, may cause further deterioration of the planarity of the substrate surface.
In addition, a curvature of the substrate of a photolithographic mask may lead to imaging errors of the mask.